MedicoPlexus

General Histology

Stephy Tom

1) Tissues – definition, classification, origin and general properties 

Tissues are groups of cells which are similar in structure and which perform common or related function.  Tissues have two interacting components: cells and extra-cellular matrix. The ECM consists of many kinds of macromolecules, most of which form complex structures such as collagen fibrils and basement membranes. The ECM supports the cells and the fluid that transport nutrients to the cells and carries away their catabolites and secretory products.  

There are four basic types of tissues: epithelial, connective, muscle and nervous tissue. Epithelial tissue is composed of closely aggregated polyhedral cells with very little extracellular substance. Its main function is lining of the surface of body cavities and glandular secretion. Connective tissue is characterized by the abundance of extracellular material produced in its cells. Its main function is for support and protection. Muscular tissue is composed of elongated cells specialized for contraction and movements. Nervous tissue is composed of cells with elongated processes extending from the cell body that have specialized for receiving, generating and transmitting nerve impulses.  

Most organs can be divided into parenchyma, which is composed of the cells responsible for the organ’s specialized functions, and stroma, the cells of which have a supporting role in the organ. Except in the brain and spinal cord, the stroma is always connective tissue.  

2) Epithelial tissue – definition, classification, origin, characteristics and function 

Epithelial tissue is composed of closely aggregated polyhedral cells with strong adhesion to one another and attached to a thin layer of ECM. Epithelia are cellular sheets that line the cavities of organs and cover the body surface. Their principal functions include protection of underlying tissue of the body from abrasion and injury, secretion of mucus, hormones, enzymes etc. from various glands, absorption of material from a lumen, detection of sensations via taste buds, retina of the eye and contractility (e.g. myoepithelial cells).  

Epithelia can be divided into two main groups: covering and secretory. Cells of covering epithelia are organized into one or more layers that cover the external surface or line the cavities of an organ. Such epithelia are classified according to the number of cell layers and the cell morphology in the surface layer. Glandular epithelium are glands which originate from invaginated epithelial cells.  

Epithelial tissues are derived from all of the embryological germ layers: from ectoderm (e.g., the epidermis of the skin), from endoderm (e.g., the lining of the gastrointestinal tract), from mesoderm (e.g. the inner lining of body cavities).  

Most epithelia rest on connective tissue that provides support and nutrition to the epithelium as well as bind it to underlying structures. Epithelial cells generally show polarity, with organelles and membrane proteins distributed unevenly within the cell. The region of the cell contacting the connective tissue is called the basal pole and the opposite end, usually facing a space, is the apical pole. The two poles of epithelial cells differ both in structure and function. Regions of cuboidal or columnar cells that adjoin the neighboring cells are the lateral surfaces; cell membranes here often have numerous infoldings to increase the area of that surface, increasing its functional capacity.  

 All epithelial cells in contact with subjacent connective tissue have at their basal surfaces a specialized, felt-like sheet of extracellular material referred to as the basement membrane. With the TEM, the basement membrane may be resolved into two structures. Nearest the epithelial basal poles is an electron-dense layer, consisting of a network of fine fibrils that comprise the basal lamina, which is produced by the epithelial cells. Beneath this layer is often a more diffuse and fibrous reticular lamina, which is produced by connective tissue cells. The basal lamina is composed of lamina lucida and lamina densa which is a meshwork of type IV collagen, coated by the proteoglycan perlecan.  Lamina lucida is made of laminin which are large glycoproteins that self-assemble as a lacelike network immediately below the cells’ basal pole where they are held in place by transmembrane integrins. The laminin network is held together by the adhesive glycoprotein entactin/nidogen.  

Basement membranes have many functions. They provide structural support and polarity to epithelial cells and attach epithelia to underlying connective tissue. Proteins of the layered meshwork help filter substances entering the epithelium from below. They also regulate mitotic activity, cell differentiation and migration.  

Epithelial cells adhere strongly to neighboring cells and basal laminae, so they are rich in intercellular junctions.  Tight junctions’ seals adjacent cells to one another, controlling passage of molecules between them and it separates apical and basal lateral membrane domains.  Adherent junctions provide points linking the cytoskeletons of adjacent cells which strengthens and stabilizes nearby tight junctions. Desmosomes provides points of strong intermediate filament coupling between adjacent cells, strengthening the tissue. Hemidesmosomes anchors cytoskeleton to basal lamina. Gap junctions allows direct transfer of small molecules and ions from one cell to another.  

3) Covering epithelia – definition, classification, distribution and histophysiology  

Cells of covering epithelia are organized into one or more layers that cover the external surface or line the cavities of an organ. Such epithelia are classified according to the number of cell layers and the cell morphology in the surface layer. Simple epithelia contain one cell layer and stratified epithelia contain two or more layers. Based on cell shape, simple epithelia are classified as squamous (thin cells), cuboidal (cell width and thickness roughly similar) or columnar (cells taller than they are wide). Most stratified epithelia are classified according to the cell shape of the superficial layers: squamous, cuboidal or columnar. The very thin surface cells of stratified squamous epithelia can be keratinized, which means filled with keratin intermediate filaments or they can be non-keratinized with relatively sparse amounts of keratin.  

Epithelial cell nuclei very in shape and may be elliptic (oval), spherical, or flattened. Nuclear shape corresponds roughly to cell shape; tall cells have elongated nuclei and squamous cells have flattened nuclei. Cuboidal cells usually have more spherical nuclei.  

Stratified squamous keratinized epithelium is found mainly in the epidermis of skin, where it helps prevent dehydration from the tissue. Its cells form many layers, with the less differentiated cuboidal cells near the underlying connective tissue. These cells become more irregular in shape and flatten as they accumulate keratin in the process of keratinization and are moved progressively closer to the skin surface, where they become thin, metabolically inactive packets of keratin lacking nuclei. This surface layer of cells helps protect against water loss across this epithelium. Stratified squamous nonkeratinized epithelium lines wet cavities (e.g., mouth, oesophagus and vagina) where water loss is not a problem. Here the flattened cells of the surface layer contain much less keratin, retaining their nuclei and metabolic function.  

Stratified cuboidal and stratified columnar epithelia are both relatively rare. Stratified cuboidal epithelium is restricted to excretory ducts of salivary and sweat glands. Stratified columnar epithelium can be found in the conjunctiva lining the eyelids, where it is both protective and mucus secreting. Transitional epithelium or urothelium lines much of the urinary tract, extending from the kidneys to the proximal part of the urethra, and is characterized by a superficial layer of large, domelike cells sometimes called umbrella cells. These cells are specialized to protect underlying tissues from the hypertonic and potentially cytotoxic effects of urine.  

In addition to the simple and stratified epithelial classifications, there is another type called pseudostratified columnar epithelium. In this case, tall, irregular cells are attached to the basement membrane, but their nuclei are at different levels and not all cells extend to the free surface, giving a stratified appearance. An example is the lining of the upper respiratory tract, where the cells are also heavily ciliated.